This invention relates to processes for manufacturing high-strength bainitic steel rails having a head surface with excellent rolling-contact fatigue resistance required of the rails used in high-speed railroads, and more particularly to high-strength rails having a bainitic structure resistant to fatigue cracks that could occur in the gage corner between the head and the sides of rails and the squat or dark spot appearing at the top plane of the rail head surface and processes for manufacturing such rails.
Recently the weight of loads carried and speed of travel have been improve to increase the efficiency of railroad transportation. Thus, railroad rails are now subjected to more severe service conditions and, therefore, required to have higher quality.
Concrete problems include a sharp increase in the wear of rails installed in curves and the incidence of fatigue crack developing from the interior of the gage corner which is the principal contact point of rails with the wheels of trains running thereover.
The following solutions have been employed for the problems just described:
(1) As rolled rails of alloyed steels prepared by adding large quantities of copper, molybdenum and other alloying elements. (Refer to Japanese Provisional Patent Publication No. 140316 of 1975.) PA1 (2) Heat-treated rails of non-alloyed steels manufactured by applying accelerated cooling (by air-mist cooling) to the head or entirety of the rail between 700.degree. and 550.degree. C. (Refer to Japanese Patent Publication No. 23885 of 1980.) PA1 (3) Heat-treated rails of low-alloy steels having improved wear and fatigue crack resistance and capability to form harder welds prepared by the addition of lower percentage of alloying elements. (Refer to Japanese Patent Publication No. 19173 of 1984.)
These high-strength rails are made of steels having bainitic, ferritic and fine-pearlitic structures to improve their resistance to wear and resistant inner fatigue defects.
In tandent and gently curved tracks of railroads where not much resistance to wear and inner fatigue defects is required, repeated contacts between wheels and rails cause rolling-contact fatigue failures on the rail head surface. This results rolling contact fatigue or transverse defects resulting from the propagation of fatigue cracks started at the top plane of the rail head surface into the interior thereof. The failures called "squat" or "dark spot" that appears mainly in the tangent tracks of high-speed railroads is a typical example. Although the occurrence of such failures has been known, conventional as-rolled rails with pearlitic structures are used in the tangent and gently curved tracks.
After a certain period of time (or after a certain tonnage of loads has been carried thereover), failures due to rolling-contact fatigue starts from the center of the rail head surface used in the tangent or gently curved tracks of railroads serving mainly for transporting passengers. Investigation by the inventors has revealed that the failures just described are due to the pile-up of damage on the center of the rail head surface that results from the repeated contacts between wheels and rails.
This failures can be eliminated by grinding the rail head surface at given intervals. However, the costs of the grinding car and operation are high and the time for grinding is limited by the running schedule of trains.
Another solution increases the wear rate of the rail head surface so that the accumulated fatigue damage were away before the defects occure. The wear rate of rails can be increased by decreasing their hardness as their wear resistance depends on steel hardness. However, simple reduction of steel hardness causes plastic deformation on the surface of the rail head which, in turn, causes head checks and other damages called flaking. Therefore, it has been difficult to effectively prevent the occurrence of the failure described above in the conventional rails of steels with pearlitic structures.